As one of processes for producing a polyolefin porous material, there is known a process for forming a large number of micropores which comprises stretching the mixture of a filler and a polyolefin to promote interfacial separation between the polyolefin and the filler and further fibrillating through the cleavage of a polyolefin phase. This process is excellent because a polyolefin porous material can be easily obtained.
For example, the present inventors have already proposed a process for producing a microporous polyolefin sheet by biaxially stretching a polyolefin sheet made from a polyolefin highly filled with a filler such as calcium carbonate or polymethyl sylsesquioxane [see Ind. Eng. Chem. Res., 32, 221 (1993)].
In the above process, the properties of the obtained microporous polyolefin sheet are determined by the type, particle diameter and amount of the filler and a stretch ratio. To obtain a microporous sheet having a smaller pore diameter, it is desirable to use a smaller filler. However, the characteristic feature of powders is such that the smaller the diameter of particles, the higher the cohesiveness of the particles become. Therefore, when a filler having a small particle diameter is blended into a polyolefin, it is difficult to disperse primary particles uniformly and the formation of agglomerates is inevitable. As a result, the size of the agglomerates affects the formation of amicroporous structure, thereby causing an increase in pore diameter and the expansion of a pore diameter distribution. Therefore, it is difficult to produce a microporous sheet having a very small pore diameter and a large pore specific surface area.
Also, the present inventors have already proposed microporous polyolefin fibers (see J. Appl. Polym. Sci. 61 2355 (1996), ibid 62 81 (1996), JP-A 7-289829, JP-A 9-157943 and JP-A 9-157944). They are microporous fibers obtained by melt-spinning and stretching a polyolefin composition containing an appropriate amount of a filler. In these microporous fibers, at least 15 wt % of a filler is required to form pores thoroughly.
In order to enhance the adsorptivity of a microporous fiber, it is desirable to decrease the diameter of pores formed in the fiber and to increase the specific surface area of each pore. Therefore, it is desirable to use a filler having as small a diameter as possible. When a filler having an average particle diameter of less than 0.1 .mu.m is used, there arises a problem such as the agglomeration of particles. This agglomeration problem becomes more serious as the amount of the filler increases as described above. When a large number of agglomerates are formed, the size of the agglomerates affects the formation of a microporous structure, thereby causing the expansion of a pore diameter distribution and making it difficult to obtain a microporous fiber which satisfies the above requirements. Further, a high-strength microporous fiber cannot be obtained owing to the agglomerates.